Electrochemical sensors are known and can be used to detect various types of gases including oxygen as well as other types of gases.
Representative sensors have been disclosed in U.S. Pat. No. 5,668,302 to Finbow et al. entitled Electrochemical Gas Sensor Assembly, issued Sep. 16, 1997, and U.S. Pat. No. 5,746,899 to Finbow et al. entitled Electrochemical Gas Sensor, issued May 5, 1998. The '302 and '899 patents have been assigned to the assignee hereof and are incorporated by reference. Useful as they have become, such sensors are not without some limitations.
A fairly common problem experienced by users of portable oxygen gas detection equipment is that the instrument can be susceptible to thermal shock and generate false alarms when the user moves between locations at different temperature. Typical conditions that might generate the false alarm condition would be when the user exits a heated office or calibration station into a cold working environment.
This situation is most noticeable in winter when the temperature difference often exceeds 30° C. Whilst the effect is often associated with a negative temperature change i.e. movement from a warm to cooler environment, the same effect can also manifest itself in the opposite sense when there is a positive temperature change.
Thermal shock in an oxygen sensor or cell is usually characterized by a rapid change in output response other than that caused by normal diffusion, when a change in temperature is experienced by the cell. Thermal shock does not always happen immediately and thermal shocks have been noticed after time periods of over one hour after the initial temperature change occurred. This has implications in a finished product of false alarms where a false oxygen level is registered by the cell.
The cause of the problem is related to the design and construction of the oxygen sensor which relies on controlled diffusion of oxygen into the sensor from the external environment via a capillary hole. Once oxygen has entered the cell it reacts and generates a current that is proportional to the oxygen concentration in the external environment.
Large temperature excursions can cause an additional contribution to the signal when the internal cell pressure (caused by the temperature change) equilibrates with the environment. The causes of these pressure differences include air, or, gas pockets within the body of the cell which expand or contract with temperature.
For the typical condition described above, the gas inside the cell contracts when the instrument is transferred to the cold environment. The pressure difference caused through contraction draws air into the cell through the capillary leading to an enhanced cell output and false alarm.
Thus, there continues to be a need for improved oxygen sensors which minimize false alarms. Preferably such improved functionality could be achieved without substantially increasing the manufacturing complexity and cost of such units.
Also, it would be preferable if such improved detectors could be implemented as portable or human wearable to facilitate use.